Convertendo um array multidimensional num único array

[Frank K Hosaka]

<?php
$mysqli=new mysqli("localhost","root","","diario");
$query=$mysqli->query("select dia,sum(custototal) as estoque from 
        tbhistprod group by dia order by dia");
$result=$query->fetch_all(MYSQLI_ASSOC);
$dias=[];
foreach($result as $key=>$value){
        $dias=array_merge($dias,[$value['dia'] => $value['estoque']]);}
if(array_key_exists('2024-02-05',$dias)){
        echo $dias['2024-02-05'];}